Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $128.44   8.216%   $138.9926   $138.99   $139.00   $139.00  
  Purchase #2 $152.34   8.754%   $165.6758   $165.68   $165.70   $165.70  
  Purchase #3 $40.74   4.639%   $42.6299   $42.63   $42.65   $42.60  
  Purchase #4 $247.96   6.271%   $263.5096   $263.51   $263.50   $263.50  
  Purchase #5 $236.47   9.616%   $259.209   $259.21   $259.20   $259.20  
  Purchase #6 $76.69   13.594%   $87.1152   $87.12   $87.10   $87.10  
  Purchase #7 $198.99   12.409%   $223.6827   $223.68   $223.70   $223.70  
  7 purchase totals = $1,081.63       $1,180.8148   $1,180.82   $1,180.85   $1,180.80